The liquid crystal display element is used in various kinds of measuring equipment, a panel for an automobile, a word processor, an electronic organizer, a printer, a computer, a TV, a clock, an advertising display board and the like including a watch and a calculator. The representative examples of a liquid crystal display method include a TN (twisted nematic) type, an STN (super twisted nematic) type, a VA (vertical alignment) type and an IPS (in-plane switching) type using a TFT (thin film transistor). The liquid crystal composition used in these liquid crystal display elements is required to be stable with respect to external factors such as water, air, heat and light, and to exhibit a liquid crystal phase in as wide a temperature range around room temperature as possible, and to have a low viscosity and a low driving voltage. Furthermore, the liquid crystal composition consists of several kinds of compounds to several dozens of compounds in order to make an optimal dielectric anisotropy (Δ∈) and/or refractive index anisotropy (Δn) be optimal values with respect to each display element.
In a vertical alignment type display, a liquid crystal composition having a negative Δ∈ is used, and the display is widely used in a liquid crystal TV. On the other hand, in all the driving methods, a low voltage driving, a high speed response, and a wide operating temperature range are required. That is to say, it is required for Δ∈ to have a positive and high absolute value, for viscosity (η) to be low, and for a nematic phase-isotropic liquid phase transition temperature (Tni) to be high. In addition, it is necessary to adjust Δn of the liquid crystal composition to an appropriate range in accordance with a cell gap by setting Δn×d which is a product of Δn and the cell gap (d). Additionally, in the case where the liquid crystal display element is applied to a TV and the like, the liquid crystal composition is required to have a low γ1 since high speed response property is emphasized.
In the related art, compounds having a dialkyl bicyclohexane skeleton were generally used in order to form the liquid crystal composition having a low γ1 (refer to PTL 1). However, bicyclohexane compounds have a high effect on reducing γ1, and generally, in compounds having a high vapor pressure with a short alkyl chain length, the tendency is particularly remarkable. Moreover, since Tni also tends to be low in the alkyl bicyclohexane compounds, compounds in which the total side chain length is equal to or more than 7 carbon atoms are used in many cases, and at the present time, compounds having a short side chain length have not been sufficiently examined.
There are compositions known as a liquid crystal composition using a dialkyl bicyclohexane compound having a short side chain length (refer to PTL 2), a compound having a three-ring structure as a compound having a negative dielectric anisotropy is widely used, and a balance of physical properties as a whole is obtained using compounds having a difluoroethylene skeleton. However, the difluoroethylene skeleton used in the composition has a problem in that the stability thereof is low with respect to light, and therefore, development of the liquid crystal composition in which such a compound is not used is desired.
On the other hand, the application of the liquid crystal display element is expanded, great changes are seen in the usage method and the preparing method thereof, and in order to cope with the changes, optimization of characteristics other than fundamental physical property values known in the related art is demanded. That is to say, as the liquid crystal display element which uses the liquid crystal composition, a VA (vertical alignment) type and an IPS (in-plane switching) type are widely used, a display element of which the size is also an extra-large size of equal to or longer than 50 inches is put into practical use and is used. With the increase in the substrate size, in an injection method of the liquid crystal composition into a substrate, the mainstream type of the injection method has been changed from a vacuum injection method in the related art to a drop injection (ODF: One Drop Fill) method (refer to PTL 3), and the problem in that dropping mark generated when the liquid crystal composition is dropped on a substrate leads to a decrease in display quality became an issue. Furthermore, a PS liquid crystal display element (polymer stabilized) and a PSA liquid crystal display element (polymer sustained alignment) are developed for the purpose of production of a pretilt angle of a liquid crystal material in a liquid crystal display element and a high speed response property (refer to PTL 4), and this problem became a bigger problem. That is to say, these display elements have a characteristic that a monomer is added to the liquid crystal composition, and the monomer in the composition is cured. Since the liquid crystal composition for an active matrix needs to maintain a high voltage holding ratio, compounds that can be used are specified, and the compounds having an ester bond in the compound is limitingly used. The monomer used in the PSA liquid crystal display element is mainly acrylate, generally, it has an ester bond in the compound, and such compounds are not normally used as the liquid crystal compound for the active matrix (refer to PTL 4). Such foreign materials induce the generation of a dropping mark, and there is a problem in that the yield of the liquid crystal display element is deteriorated by a display defect. In addition, even when additives such as an antioxidant, and a light absorber are added in the liquid crystal composition, there is a problem in that the yield is deteriorated.
Here, the dropping mark is defined as a phenomenon in which, in the case where a black color is displayed, a white mark formed by dropping of the liquid crystal composition comes up to the surface.
For the suppression of the dropping mark, a method in which the dropping mark which is generated in relation to the alignment control film by forming a polymer layer in the liquid crystal layer by polymerization of polymerizable compound mixed in the liquid crystal composition is suppressed is disclosed (refer to PTL 5). However, in this method, there is a problem of burn-in of the display due to the polymerizable compound added to the liquid crystal, and the effect for suppressing the dropping mark is also insufficient. And therefore, development of the liquid crystal display element in which the fundamental characteristics of the liquid crystal display element are maintained, and the burn-in and the dropping mark are unlikely to be generated is required.